Overcurrent exceedsRated currentCurrent.The circuit current greater than the rated current carrying capacity of the circuit conductor is overcurrent.It includesOverload currentandshort-circuit current。The difference is that the overcurrent before the circuit insulation is damaged is called overload current;Overcurrent after insulation damage is called short circuit current.
In aPower system，OvercurrentorExcess currentIs the larger one than expectedelectric currentEquipment that exists through conductors, thus causing excessive heating, and risk of fire or damage.Possible causes of overcurrent include short circuit, excessive load, incorrect design or ground fault.Usually usedFuse，Circuit breaker，Temperature sensorandCurrent limiterTo control overcurrentrisk。
Main categories
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Overload current
The electrical circuit is connected to too many electrical equipment or supplied equipmentoverload(For example, the mechanical load of the connected motor is too large).Its current value is only a few times of the current carrying capacity of the circuit. The consequence is that the operating temperature exceeds the allowable value, which accelerates the deterioration of insulation and shortens its life. It does not directly cause disasters.[1]
short-circuit current
When the circuit insulation is damaged due to various reasons (including overload), conductors with unequal potentialimpedanceNegligible fault points are connected, which is calledshort circuit。Since the paths of this short circuit circuit are all metal paths, this short circuit is classified asMetallic short circuitIts short-circuit current value can reach hundreds to thousands of times of the current carrying capacity of the circuit conductor, which can produce abnormal high temperature or huge mechanical stress, thus causing various disasters.
In order to protect the circuit from overload protectionoverloadThe protective apparatus and the protected circuit shall cooperate on some parameters, and they shall meet the following conditions:
(1) Rated current orSetting currentInIt shall not be less than the calculated load current I of the circuitB。
(2) Rated current or setting current I of protective apparatusnIt shall not be greater than the allowable continuous current carrying capacity of the circuit (hereinafter referred to asCurrent carrying capacity）。
(3) Current I to ensure effective action of protective apparatustwoIt shall not be greater than 1.45 times of the loop current carrying capacity.
The above conditions are expressed in the formula: IB≤In≤IZAnd Itwo≤1.45IZ
Where: IB- Load current of the circuit (A);
In- Rated current of fuse or circuit breaker or setting current (A);
IZ- Current carrying capacity of circuit conductor (A);
Itwo- Current to ensure effective action of protective appliances (i.e. fusing current or tripping current) (A).[2]
Current quick break
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The overcurrent of electric equipment is a form of fault. When the overload is not serious, it can not be cut off immediately. The other one has a longer delay. If the overload is more serious, the delay will be shorter, which is equivalent to time limitCurrent quick break protectionIf the overload is particularly seriousshort circuitThe transformer must immediately remove the fault, which is the instantaneous current quick break protection, so the current quick break can be understood as particularly largeOvercurrent protection, that is, the overload is particularly serious, and the electrical equipment must be disconnected from the system immediately, which isCurrent quick break protection。The current quick break cannot protect the whole length of the line, because the current quick break protection is set to avoid the current in case of terminal fault of the protected line. Since it is avoided, the whole length cannot be protected. The purpose of setting the current in case of terminal fault of the line is to prevent the protection from bypassing the level and misoperation in case of fault at the head end of the next level line,Because the fault at the head end of the next level shall be removed by the current quick break protection of the next level.In case of the head end fault of the next level and the end fault of the line, the short circuit current is almost equal and cannot be distinguished.
When the line is long, there are two cases of short circuit in the line. One is short circuit at the head end: the current is very large, which is not allowed. It is called quick break current (generally more than five times the rated current). The quick break protection current is implemented without time limit.The other is short circuit at the end of the line: because the line is long, has a certain impedance, and the short circuit current is large (about 3-5 times the rated current), it is called overcurrent. Although the current is large, it will not burn out the line equipment within 0.2-2 seconds. Therefore, overcurrent protection is set in this case, and the time limit is set within 2 seconds. If the time limit is exceeded, the line and equipment will be tripped, and false faults can also be distinguished,Such as short circuit caused by strong wind and lightning.[3]
Current quick break means that when the current reaches this value, it will trip or extend for a short time, and then trip soon.Because the current quick break setting value is too large, when the line is too long, the short circuit current at the end cannot reach the tripping value due to the high resistance of the line, so it is not necessary to trip, so the full length of the line cannot be protected.The tripping current set for overcurrent protection is smaller than the current quick break, and it will take a certain time to reach the tripping value. If the current is still large, it will trip.relay protectionIt is a systematic project, and multiple protections are completed in coordination. It not only avoids lightning and other instantaneous large currents, but also can distinguish the fault location. A certain range is tripped by a certain switch;This requires setting different tripping current values with different tripping times.The next level and this line are usedCircuit breakerThe circuit has a new setting after passing a circuit breaker.
Overload
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Overcurrent refers to the fault of the line, but it is not within the protection range of quick break, generally single-phase grounding, etc., which requires tripping within a certain time.
Overload refers to the excessive load of the line, which is not a fault but an alarm signal. It usually occurs in summer, when the factory starts, and when everyone is using air conditioners, it exceeds the rated value of the equipmentload。When the load is too large, the service life of the primary equipment will be shortened, especially the transformer overload.
The two have different protection purposes,Overcurrent protectionFor the protection against short-circuit fault, overload is essentially for the thermal capacity of the equipment itselfHigh voltage transmission line, or for system stability.The overcurrent protection shall be coordinated with the protection section for short circuit fault in the adjacent device, and the overload shall be coordinated with the thermal load capacity of the equipment.[4]
The action outlets of the two are also different. The overcurrent protection is aimed at short circuit fault, and the final outlet is to isolate the protection range. For the line, sometimes the reclosing will be started;In case of overload, reclosing will not be started generally, and sometimes there may be a design of joint switching.
There are two main differences between overcurrent protection and overload protection: one is that the setting action value is different, and the other is that the action time is different.
The setting value of overload protection is much lower than that of overcurrent protection, but it should be higher than the normal load to prevent the equipment from overload operation, and a certain time delay should be added to avoid the starting current of the equipment.When the overcurrent protection refuses to operate for some reason, the overload protection can also be used as the overcurrent protectionBackup protection。
